Overhead doors in which a series of door panels are hinged together, each panel having steel inner and outer skins and a foam insulating core, are well known in the art. Typically, such panels are manufactured by foaming the cores in place, usually in continuous production runs where one web of steel is unrolled from a supply roll, advanced through a forming station where its sides are folded inwardly, and coating with a liquid which upon further treatment and heat becomes the plastic foam core. A similar web of steel is unrolled from another supply roll to become the opposite skin. The inturned sides of the two steel webs are sometimes joined directly together but, more frequently, are connected to the core but not directly to each other so that a direct path of thermal conductivity between the skins is avoided. The spaces between the two skins are often bridged by resilient spacers that serve to provide a "thermal break" as well as a protective joint seal between the skins. A common characteristic of such constructions is that where a thermal break is provided the inner and outer metal skins are seldom if ever physically interlocked together. The integrity of the steel-foam-steel sandwich of such a panel is therefore maintained primarily by the bond formed between the inner surfaces of the skins and the plastic material of the core. Reference may be had to the following patents illustrative of the prior art: Nos. 4,183,393, 4,123,885, 3,336,713 (metal skins spaced apart from each other and bonded to foam cores); No. 3,786,613 (skins bonded to and interlocked with foam core without thermal break); U.S. Pat. Nos. 3,992,837, 3,830,027, 2,008,325 (inner and outer skins joined together directly or by means of splines without thermal break).
In the earlier constructions in which the side edges of the inner and outer metal skins are joined directly together, or are coupled by splines or other connecting strips, the requirements of strength, rigidity, and durability have been achieved at the expense of poor thermal barrier performance. More recent constructions in which the side edges of the skins are spaced apart, even though they may be bridged by joint seals of low thermal conductivity, function more effectively as thermal barriers but sacrifice long-term durability and strength. Over extended periods, some foam disintegration or weakening necessarily occurs with such changes often being accelerated by the gradual escape of inert gases from the foam structure and the replacement of such gases by outside air. As degradation of the foam core occurs, the forces maintaining the integrity of such a panel are diminished. Such problems are more serious where the plastic cores are foamed in place since consistency of cellular structure and uniformity of contact between the foam and the inner surfaces of the skins are difficult to achieve and control. The presence of voids or spaces where foam may be absent within the panel cannot be readily determined and such voids obviously reduce the strength of such a panel and increase the possibilities of delamination and failure.